1. Field of the Invention
This invention relates generally to a metal complex composition and, more specifically, to a RExe2x80x94Baxe2x80x94Cu (RE: a rare earth element) complex composition suitable as a precursor of a superconducting material such as Y-123 and Y124. The present invention is also directed to a process for the fabrication of a superconductor by a coating method.
2. Description of Prior Art
U.S. Pat. No. 5,039,654 (filed Nov. 9, 1987 by Mizuta et al) discloses a method of preparing a superconductive material having a metal oxide superconductor layer provided on a substrate. The method includes dissolving a blend of organic group-containing metal compounds in an organic solvent to form a precursor solution containing metal constituents in a stoichiometry nearly equal to that of the final metal oxide superconductor, and applying the precursor solution on the substrate. The coating is then dried and calcined to form the metal oxide superconductor layer. The above method, which is simple and economical and can produce an oxide film having a uniform thickness, a uniform composition and a large size, has been also utilized for the formation of an epitaxial metal oxide film on a single crystal substrate.
Similar techniques have been thereafter proposed by A. Gupta et al (Appl. Phys. Lett., 52 (1988) 2077, by P. C. McIntyre et al (J. Mater. Res., 5 (1990) 2771) and by Superconductivity Research Laboratory of International Superconductivity Technology Center, in which an organic solvent solution of trifluoroacetic acid salts of metals is used as a precursor coating liquid and in which calcination of the coating is performed in a steam-containing atmosphere. It is considered that a precursor of fluorine-containing components produced during calcination serves to facilitate the formation of a highly oriented superconductive phase.
It has been found that the coating liquid containing trifluoroacetic acid salts of metals has a problem because a substrate made of a metal or a metal oxide such as nickel, silver, nickel oxide or magnesium oxide is corroded with the coating liquid having a strong acidity. Additionally, when the coating and subsequent pre-calcination are repeated several times to form a coating having a relatively large thickness, the pre-calcined coating is apt to be dissolved in the newly applied coating liquid. The corrosion and dissolution adversely affect the smoothness of the superconductive film as well as uniformity in composition of the film.
It is, therefore, the primary object of the present invention to provide a metal complex composition which is useful as a raw material for the production of a Y123-type or Y124-type superconductive phase.
Another object of the present invention is to provide a metal complex composition of the above-mentioned type which, in the form of an organic solvent solution, is neutral and stable and does not corrode or dissolve a metal or a metal oxide.
It is a further object of the present invention to provide a metal complex composition which can form a large thickness superconductive layer on any conventionally employed substrate.
In accomplishing the foregoing object, there is provided in accordance with the present invention a metal complex composition which comprises complexes of metal species including a rare earth element, barium and copper, wherein the complexes have ligands of (a) trifluoroacetic acid or pentafluoropropionic acid, (b) pyridine and (c) acetylacetone coordinated with the metal species.
In another aspect, the present invention provides a method of preparing the above metal complex composition, comprising the steps of:
(a) providing a solution comprising (A) a rare earth element salt selected from the group consisting of an acetylacetonate, a trifluoroacetate and a pentafluoropropionate, (B) a barium salt selected from the group consisting of an acetylacetonate, a trifluoroacetate and a pentafluoropropionate and (C) a copper salt selected from the group consisting of an acetylacetonate, a trifluoroacetate and a pentafluoropropionate dissolved in a mixed solvent including pyridine and at least one member selected from the group consisting of acetylacetone, trifluoroacetic acid and pentafluoropropionic acid such that said solution contains acetylacetone and at least one of trifluoroacetic acid and pentafluoropropionic acid, and
(b) removing the mixed solvent from said solution to obtain the above metal complex composition as a solid phase.
In a further aspect, the present invention provides a method of preparing the above metal complex composition, comprising the steps of:
comprising providing an acidic solution of trifluoroacetic acid or pentafluoropropionic acid salts of a rare earth element, barium and copper dissolved in an organic solvent, and mixing said solution with pyridine and acetylacetone to obtain the above metal complex composition in the form of a neutral solution.
The present invention also provides a process for the preparation of a superconductor, comprising the steps of:
(a) applying an organic solvent solution of the above metal complex composition to a substrate to form a coating thereon; and
(b) heat treating said coating at a temperature and for a period of time sufficient to form a superconductive phase.
Trifluoroacetic acid or pentafluoropropionic acid has two oxygen atoms which can be coordinated to the same metal ion or two different metal ions. The pyridine ligand has one nitrogen atom in the pyridine ring which can be coordinated to one metal ion. Acetylacetone has two oxygen atoms which can be coordinated to the same metal ion or two different metal ions. Further, one or a plurality of ligands may be coordinated to the rare earth element ion, barium ion and copper ion. For example, one rare earth element ion can be coordinated with at most three pyridine ligands, at most six different ligands of acetylacetone, trifluoroacetic acid or pentafluoropropionic acid.
Thus, in the composition of the present invention, two or more, same or different metal ions may be indirectly linked at random to each other through the trifluoroacetic acid or pentafluoropropionic acid ligand and the acetylacetone ligand. The fact that a solution of the composition does not segregate crystals but forms an amorphous solid (glass) when the solvent is removed to dryness is considered to be ascribed to the above coordination structure which prevents regular arrangement of metal species.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the invention to follow.